As an optical device such as a laser device or an optical amplification device, a fiber optical device is known that uses an amplification optical fiber having a laser active substance such as Yb added thereto. In this type of optical device, with the active substance excited by supplying excitation light (for example, light with a wavelength of 975 nm) to the amplification optical fiber, seed light (light to be amplified) is input to the optical fiber, and thus the seed light is amplified by induced emission and the resulting light is output as output light from the optical fiber. When the optical device is configured as a laser device, a resonator is provided for the amplification optical fiber, and the amplified light performs lasing within the resonator, with the result that output laser light is generated.
Currently, fiber optical devices such as a Yb-doped fiber laser device are widely used for example as an industrial continuous operation type (continuous oscillation type) laser light source and the like. On the other hand, in a pulse operation type fiber optical device, an optical fiber deterioration phenomenon called photodarkening disadvantageously occurs, resulting from the supply of excitation light to the amplification optical fiber, over a wide range from an ultraviolet region to a near-infrared region (for example, see Non Patent Document 1). When the photodarkening occurs, a light transmission loss in the amplification optical fiber is increased.